One female protagonist will generally not spur a femslash fandom, because a majority of texts with a female protagonist will still show her interacting primarily with male secondary characters (too many examples to count)

Childhood classics where girls-as-best-friends is the rule rather than the exception (Anne/Diana in Anne of the Green Gables and many others)

A sufficiently large cast, especially in book, comics or animates canons, will generally spur a profusion of minor femslash pairings between minor characters, but will usually not create an overwhelming femslash fandom, unless a pairing gains unusual momentum.